1. Field of the Invention
The present invention relates to surgical apparatus and more particularly to a neuroelectrophysiological monitoring instrument for use in conjunction with one or more electrical stimulus probes as an intraoperative aid in defining the course of neural structures. The invention is particularly applicable for use in monitoring facial electromyogram (EMG) activity during surgeries in which a facial motor nerve is at risk due to unintentional manipulation and will be described with reference thereto, although it will be appreciated that the invention has broader applications and can be used in other neuroelectrophysiological monitoring procedures.
2. Discussion of the Prior Art
Despite advances in diagnosis, microsurgical techniques, and neurotological techniques enabling more positive anatomical identification of facial nerves, loss of facial nerve function following head and neck surgery such as acoustic neuroma resection remains a significant risk. Nerves are very delicate and even the best and most experienced surgeons, using the most sophisticated equipment known, encounter a considerable hazard that a nerve will be bruised, stretched or severed during an operation. Studies have shown that preservation of the facial nerve during acoustic neuroma resection may be enhanced by the use of intraoperative electrical stimulation to assist in locating nerves. Very broadly stated, the locating procedure, also known as nerve integrity monitoring, involves inserting sensing or recording electrodes directly within cranial muscles innervated or controlled by the nerve of interest. A suitable monitoring electrode is disclosed in U.S. Pat. No. 5,161,533 (to Richard L. Prass et al.), the entire disclosure of which is incorporated herein by reference.
Electrical stimulation is then applied near the area where the subject nerve is believed to be located. If the stimulation probe contacts or is reasonably near the nerve, the stimulation signal applied to the nerve is transmitted through the nerve to excite the related muscle. Excitement of the muscle causes an electrical impulse to be generated within the muscle; the impulse is transferred to the recording electrodes, thereby providing an indication to the surgeon as to the location of the nerve. Stimulation is accomplished using hand held probes such as the Electrical Stimulus Probe disclosed in U.S. Pat. No. 4,892,105 (to Richard L. Prass), the entire disclosure of which is incorporated herein by reference. The probe of Pat. No. 4,892,105 has become known as the Prass Flush-Tip Monopolar Probe and is insulated up to the distal tip to minimize current shunting through undesired paths. An improved structure for a bipolar probe is disclosed in patent application Ser. No. 09/362,891 entitled Bipolar Electrical Stimulus Probe, the entire disclosure of which is also incorporated herein by reference.
Prior art nerve integrity monitoring instruments (such as the Xomed.RTM. NIM-2.RTM. XL Nerve Integrity Monitor, manufactured by the assignee of the present invention) have proven to be effective in performing basic functions associated with nerve integrity monitoring such as recording EMG activity from muscles innervated by an affected nerve and alerting a surgeon when the affected nerve is activated by application of a stimulus signal, but are not suitable for some surgical applications and present difficulties in certain environments.
The Xomed.RTM. NIM-2.RTM. XL Nerve Integrity Monitor (or NIM-2XL monitor) is powered by specially adapted battery packs for low noise operation. The 110 Volt Alternating Current (AC) mains supply is a well known source of 60 Hz him, RF interference and voltage spikes, and so the designers of the NIM-2XL monitor opted to avoid introduction of any possible interference in the highly sensitive low noise amplifiers required to sense microvolt (.mu.V) level nerve monitoring signals by relying on battery power. In day-to-day practice, however, the battery packs are not always sufficiently charged to operate the NIM-2XL monitor for the required length of time; alternatively, the user may not have a sufficient quantity of fully charged battery packs for a lengthy procedure, since the staff may forget to recharge depleted battery packs. In response to these difficulties, users have asked for a nerve integrity monitor that can be connected to the 110V AC mains supply; those users still require highly sensitive nerve integrity monitoring circuitry, however.
The NIM-2XL monitor provides two channels of EMG monitoring, a sufficient configuration for at least 90% of the head and neck procedures performed, at present. It is anticipated that in the future, however, four or more channels of monitoring might be required for perhaps one in three head and neck procedures. This presents an economic difficulty since the NIM-2XL monitor is an expensive instrument and very few medical facilities could muster two or more NIM-2XL monitors (with fully charged battery packs) in response to this anticipated need.
Another problem arises in foreign countries which often lack surgical staff trained in the English language. The NIM-2XL monitor includes user screens with extensive directions for electrode placement and the like, all written in English, and so is not particularly well suited to use in non-English speaking environments.
Users have also requested improved EMG monitoring sensitivity. At present, the NIM-2XL monitor provides the ability to distinguish EMG events having an amplitude of approximately 25 .mu.V; it would be desirable to provide sensitivity and noise rejection allowing one to distinguish events having an amplitude of 10 to 15 .mu.V. Highly sensitive signal amplifiers are difficult to isolate from 60 Hz hum, RF noise and interference, however, and, as noted above, are often not compatible with power supplies connected to a noisy 110V AC mains supply. If additional sensitivity and noise rejection were provided however, the users would be able to detect smaller myogenic events than is currently possible.
Users have also noted that monitoring of probe continuity presents another set of problems. Periodic checks for probe continuity do not provide timely alerts to the surgeon or OR staff, in the event that probe continuity is interrupted during a procedure.
Users have noticed that certain kinds of artifacts have a disruptive effect on monitoring and tend to cause undesirable false alarms. In particular, EMG monitoring often is performed with electrocautery in a surgical procedure, wherein powerful currents surge through and cauterize the tissue, often to devastating effect on the monitor's sensitive amplifier circuits; electrocautery can also induce an undesired DC offset from buildup of charge on the monitoring or sensing electrodes. Additionally, pop noise (in the form high frequency spikes) is observed when non-insulated instruments are accidentally brought into contact, thereby providing threshold triggering false alarm. Often. false alarms are attributable to changes in what may be deemed background noise, which may be falsely identified as genuine EMG signals. Monitoring time is reduced by undesired and distracting false alarms, and so any solution to false alarms would increase the accuracy and effectiveness of the monitoring procedure.
Often, other sensitive electronic instruments are in use when intraoperatively monitoring neuroelectrophysiological signals in the body and these other instruments are also likely to be disrupted by electrocautery or the like. It may not be possible to clamp multiple muting detector sensors near the electrocautery instrument, and so clutter becomes a problem when performing delicate surgery in the head or neck.
Others have noted the problem in detecting true EMG signals even when muting and other sources of interference don't present a problem. In particular, authors Daniel M. Schwartz and Seth I. Rosenberg identified spurious activity when sensing a recording channel and dubbed it "artifact contamination" from the "antenna effect" created by placement of numerous hemostats in the operative field. Additional electrodes including a "control" electrode were described for use in method to improve spatial selectivity while discriminating between true EMG and the undesired "artifacts". Schwartz and Rosenberg's description of facial nerve monitoring is included as Chapter 6, pages 121 through 130, of the text Neuromonitoring in Otology and Flead and Neck Surgery, edited by J. Kartush and K. Bouchard, Raven Press, Ltd., 1992, the entirety which is incorporated herein by reference. Schwartz and Rosenberg have identified one method to discriminate spurious artifact signals from desired EMG signals in the innervated muscle using a signal provided by the control electrode spaced far away from the EMG electrode. In brief, if activity in the control channel is present, then it is deemed to be a product of artifact contamination.
A problem associated with using Schwartz and Rosenberg's suggested method is that monitoring instruments of the prior art customarily include two or fewer channels and so the controlled channel must somehow be configured to use a channel ordinarily reserved for an EMG sensing electrode. This problem confounds the above mentioned difficulties with properly placing electrodes and maintaining the electrical continuity of electrodes in the body. If an electrode falls out or if an electrode is misidentified as being either the control electrode or the innervated muscle EMG electrode, then that confusion may lead to an error during the procedure.
Turning to another concern, nerve integrity monitors of the prior art require the user to perform periodic checks before use, to make sure that the monitor is operating properly; as an example, an electrode imbalance check must be performed periodically to make sure that electrode placement is proper when imbalance is greater than, e.g., 20% of the greatest electrode impedance. Each check takes valuable time. The surgical staff does not have time to spare, since operating room time is exceedingly expensive and the user often has to set the monitoring instrument up in a selected configuration for a specific procedure, before beginning surgery.
There is a need, then, for a nerve integrity monitoring instrument having greater flexibility in use, greater sensitivity and noise rejection, enhanced ability to help resolve differences between genuine EMG signals and background noise and, as always, the lowest possible manufacturing cost.